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<title>The C Preprocessor: Diagnostics</title>

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<a name="Diagnostics"></a>
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<p>
Next: <a href="Line-Control.html#Line-Control" accesskey="n" rel="next">Line Control</a>, Previous: <a href="Conditionals.html#Conditionals" accesskey="p" rel="prev">Conditionals</a>, Up: <a href="index.html#Top" accesskey="u" rel="up">Top</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p>
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<hr>
<a name="Diagnostics-1"></a>
<h2 class="chapter">5 Diagnostics</h2>
<a name="index-diagnostic"></a>
<a name="index-reporting-errors"></a>
<a name="index-reporting-warnings"></a>

<a name="index-_0023error"></a>
<p>The directive &lsquo;<samp>#error</samp>&rsquo; causes the preprocessor to report a fatal
error.  The tokens forming the rest of the line following &lsquo;<samp>#error</samp>&rsquo;
are used as the error message.
</p>
<p>You would use &lsquo;<samp>#error</samp>&rsquo; inside of a conditional that detects a
combination of parameters which you know the program does not properly
support.  For example, if you know that the program will not run
properly on a VAX, you might write
</p>
<div class="smallexample">
<pre class="smallexample">#ifdef __vax__
#error &quot;Won't work on VAXen.  See comments at get_last_object.&quot;
#endif
</pre></div>

<p>If you have several configuration parameters that must be set up by
the installation in a consistent way, you can use conditionals to detect
an inconsistency and report it with &lsquo;<samp>#error</samp>&rsquo;.  For example,
</p>
<div class="smallexample">
<pre class="smallexample">#if !defined(FOO) &amp;&amp; defined(BAR)
#error &quot;BAR requires FOO.&quot;
#endif
</pre></div>

<a name="index-_0023warning"></a>
<p>The directive &lsquo;<samp>#warning</samp>&rsquo; is like &lsquo;<samp>#error</samp>&rsquo;, but causes the
preprocessor to issue a warning and continue preprocessing.  The tokens
following &lsquo;<samp>#warning</samp>&rsquo; are used as the warning message.
</p>
<p>You might use &lsquo;<samp>#warning</samp>&rsquo; in obsolete header files, with a message
directing the user to the header file which should be used instead.
</p>
<p>Neither &lsquo;<samp>#error</samp>&rsquo; nor &lsquo;<samp>#warning</samp>&rsquo; macro-expands its argument.
Internal whitespace sequences are each replaced with a single space.
The line must consist of complete tokens.  It is wisest to make the
argument of these directives be a single string constant; this avoids
problems with apostrophes and the like.
</p>



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